1. Field of the Invention
The present invention relates to a temperature-dependent self-refresh timing circuit for a semiconductor memory device.
2. Description of the Related Art
Semiconductor memory devices are widely used in many electronic products and computer systems to store and retrieve data. A semiconductor memory device comprises a large number of memory cells, each of which is constructed of one transistor and one capacitor. A DRAM memory device achieves storage of data by storing charges in capacitors. Because a charge stored in a capacitor slowly leaks over time, the memory cells of the DRAM memory device need to be periodically refreshed or updated to a full value.
Several schemes have been developed to periodically refresh memory cells in a DRAM memory device, one of which is a self-refresh scheme. The self-refresh is a mode in which a memory cell corresponding to an address generated by an internal address counter performs a refresh operation in a predetermined period when a self-refresh command is input. The predetermined period is typically determined according to a data maintenance time in the DRAM cell. After the refresh operation, the address counter is re-initialized and is provided for the next self-refresh command.
Typically, the self-refresh mode is set in a low power consumption mode and a current consumption in the self-refresh mode is required to be as low as possible. One way to reduce power consumption required for the self-refresh in the DRAM memory device is to change the predetermined refresh period in response to the ambient temperature. That is, the refresh operation may be performed in a longer predetermined period when the temperature is less than a desired value, while alternatively the refresh operation may be performed in a shorter predetermined period when the temperature is larger than the desired value.
In order to sense the ambient temperature, a temperature sensor is placed in a DRAM device to provide a corresponding temperature signal, and the predetermined period is varied in response to the temperature signal. However, in the conventional art, the temperature sensor is always activated to detect the temperature, thereby increasing the total power consumption of the DRAM memory device. In order to reduce the power consumption, there is a need to provide a timing circuit to control the predetermined period of time and to provide an enable circuit to selectively activate the timing circuit.